Low-energy circuit contacts must be of low and stable contact resistance and this can be assured only if the contact metal is a good conductor and does not tarnish with time. The noble metals, such as gold, and the metals of the platinum family which have very low chemical reactivity and essentially do not oxidize or form sulfides meet the foregoing requirements.
Due to the cost of the noble metals, low-energy circuit contacts are not made entirely of noble metals but, rather, the noble metal is electrodeposited on a base metal substrate. At times, where needed, a circuit contact will be plated with two or more noble metals and/or metals of the platinum family in sequence, such as gold over a base layer of palladium. These deposits must be essentially pore-free to prevent foreign matter from entering the pores and spreading onto the contact surface. Porous deposits cause films to be formed on the contacts. These films are produced by corrosion products which result either from the tarnishing of the base metal substrate or from direct-couple corrosion between the base and noble metals.
Gold has been widely used for low-energy circuit contacts since it has excellent resistance to chemical attack and is less expensive than any of the platinum metals with the exception of palladium. However, gold is soft and the common electrodeposited gold alloys suitable for use in low-energy circuit contacts have relatively poor resistance to wear. Palladium, because it is less expensive than gold and is a relatively reactive member of the platinum family, can effectively replace gold for some contact applications. Also, palladium wears better than gold. Further, the density of palladium is lower than the density of gold; thus, for equal thickness, the relative expense of the same thickness of metal contact can be decreased by a factor of two. Where an external gold layer is desired, advantages can be obtained by applying a base layer of palladium as a portion of the total thickness.
Printed circuit cards, that is, cards on which printed circuits are formed, have heretofore used palladium in their electrical contacts for connecting to external circuitry. Assignee's U.S. Pat. No. 3,150,065, which issued Sept. 22, 1964, disclosed a method which employs a palladosammine chloride bath for plating palladium on the electrical contacts of a printed circuit board. This patented method is most widely used in the barrel plating of palladium on electrical contacts which have a pin configuration. The bath required no additives and only minor modifications to maintain a level of plating quality suitable to produce a finished pin. Further, commonly assigned U.S. Pat. No. 3,920,526, issued Nov. 18, 1975, discloses a method for electrodeposition of palladium which employs a palladosammine chloride plating bath containing 16 to 32 grams per liter (g/l) palladosammine chloride, 65 to 250 grams per liter ammonium chloride and sufficient aqueous ammonia to maintain the pH of the plating bath at least 8.8.
In today's technology, the printed circuit cards and the modules to which they are connected have become more complex and it has become necessary to palladium plate electrical contacts or connectors, such as frames, flanges, connector pins, spring contacts and the like, which have an irregular shaped configuration. Also, with the increase in volume of usage of such contacts a high speed plating operation is desirable wherein the contacts are processed in rack or strip form. When the plating bath of the above-mentioned U.S. Pat. No. 3,150,065 patent ws tried out for this mode of operation, it was found to have some unsatisfactory limitations. In order to maintain the desired operation current density range of 3-30 amps/ft.sup.2 and more particularly 15-25 amps/ft.sup.2, it was necessary to employ a high concentration of palladium in the bath which resulted in drag out and a waste of palladium. Also, due to the low chloride content, the bath was not sufficiently conductive and ductile for a high speed rack-type of operation. The plating deposits turned out to be dull, multi-shaded and non-uniform which is totally unacceptable for a contact surface finish. In addition, the process of the U.S. Pat. No. 3,920,526 case leads to unacceptable products when used for electrical parts of relatively complex configurations plated at high speed.
U.S. Pat. No. 3,637,474, issued on Jan. 25, 1972 to Zuntini et al. and assigned to the Sel-Rex Corporation, discloses an electroplating bath for the deposition of palladium from a palladium-urea complex one example of which includes sulfite ions derived from sodium sulfite in solution in the bath. However, this process must be carried out at an elevated temperature (50.degree.-55.degree. C) and requires relatively high sulfite ion concentrations in excess of 2000 parts per million. Furthermore, the Zuntini et al. reference apparently has an upper current density of about 10 amps/ft.sup.2.
Attempts to utilize the sulfite ion concentrations disclosed by Zuntini et al. in applicants' process resulted in a heavy white precipitate in the bath which caused the plating process to be inoperative for the intended purpose. While the reason for this phenomenon is not known, it became noticeable at sulfite ion concentrations of about 2000 ppm.
Other palladium processes available in the market were also tried out but these resulted in cracked palladium and adhesion problems from high stresses, poor chemical stability of baths and replenisher solutions and poor reproducibility. It became apparent that an improved palladium plating bath solution would have to be developed which would be capable of high speed rack plating of parts and particularly those having an irregular shaped configuration.